The present invention relates to a method of fabricating a turbine engine drum, and to such a drum forming part of a low-pressure turbine.
In the prior art, a low-pressure turbine drum comprises at least two rotor disks on a common axis that are connected to each other by a substantially cylindrical or frustoconical wall forming a body of revolution that is coaxial about the common axis. Each drum has an upstream flange and a downstream flange for fastening the drum to upstream and downstream drums, respectively. The radially outer periphery of each disk has slots for axially receiving and radially retaining the roots of rotor blades.
Radially extending wipers are formed on the outer surface of the wall and they co-operate by rubbing against blocks of abradable material to form labyrinth type seals. These blocks of material are carried by the radially inner peripheries of stator vanes that are interposed between the rotor blades.
The low-pressure turbine drum may be made by welding an upstream disk to a downstream disk carrying the wall by using inertial friction welding. The first step consists in forming the slots in the outer periphery of the upstream disk, and then a second step consists in setting the upstream disk carrying the wall into rotation and in applying axial force to the second disk so that the free end of the wall becomes welded with the first disk by inertial friction welding. In a last step, the slots are made in the outer periphery of the second disk by broaching.
In order to improve the performance of the turbine and reduce its sound emission, it is known to establish multi-stage aerodynamic coupling between two consecutive sets of rotor blades that are spaced apart from each other by a set of stator vanes (a technique known as “clocking”). This coupling consists in angularly positioning the set of blades on the downstream rotor relative to the set of blades on the upstream rotor in such a manner that the wakes formed at the trailing edges of the upstream blades impact against the leading edges of the downstream blades with some specified tolerance. Thus, so far the drum is concerned, this leads to the slots in the downstream disk being broached in such a manner that they are offset in a circumferential direction through a certain angle relative to the slots in the upstream disk. Such aerodynamic coupling is described in detail in prior patent application FR 07/08710 in the name of the Applicant.
Nevertheless, such determined angular positioning of the two disks of a drum is possible only when the outer periphery of the downstream disk presents a diameter that is significantly greater than that of the upstream disk so that it is possible to perform broaching therein after welding. In configurations in which both disks have substantially the same diameter, it is impossible to perform broaching in the second disk after it has been welded because of the length of the broaching tool. Under such circumstances, the broaching operation must be performed before performing friction welding, and it is no longer possible to achieve accurate angular positioning of the slots in the second disk relative to the slots in the first disk.
Furthermore, in the prior art, the wall forming a body of revolution is made of the same material as the disks, e.g. such as NC19FeNb, which means that it is not possible to achieve accurate control over the collapse of material in the weld zone (i.e. the quantity of material that deforms under the effect of temperature). This leads to inaccuracy in the relative axial positioning of the two disks.